Image forming apparatuses such as a digital copy machine, a laser printer, and a facsimile machine are prevailing. An optical scanning unit having a laser scanning optical system that scans a laser beam is used for the above image forming apparatus. When an image is formed in the image forming apparatus, after a photoreceptor drum that is an image carrier is charged by a charging unit, writing according to image information is executed by the optical scanning unit and an electrostatic latent image is formed on the photoreceptor drum. The electrostatic latent image on the photoreceptor drum is visualized by toner supplied from a development unit. The toner image visualized on the photoreceptor is transferred onto a recording paper sheet by a transfer unit and is fixed on the recording paper sheet by a fixing unit. Thereby, a desired image is obtained.
To an image forming apparatus using the above optical scanning unit, higher speed operations and larger capacity information processing are demanded. To realize such a demand, for example, it can be contemplated to employ a configuration to increase the rotation speed of a polygon mirror and a pixel clock frequency.
An optical scanning unit is practically used that is adapted to be able to expose simultaneously a plurality of scanning lines by applying and scanning a plurality of laser beams (a multi-beam) simultaneously to/on a photoreceptor drum to improve the image forming speed. The writing speed can be increased by writing simultaneously a plurality of scanning lines using the multi-beam.
At present, the rotation speed of the polygon mirror and the pixel clock frequency as above are usually set respectively to values that are each close to the limit value thereof and increasing the rotation speed and the pixel clock frequency any more is practically difficult.
In contrast, the above scheme of applying simultaneously the plurality of laser beams to the photoreceptor drum using the multi-beam is highly reliable because image forming can be executed at a high speed even with high resolution and, therefore, this scheme is employed especially in an image forming apparatus of high-speed specifications.
In an optical system to expose simultaneously a plurality of scanning lines using a plurality of laser beams, a multiple-emission laser diode (LD) having a plurality of emitting points is used as a laser light source thereof. For example, Japanese Laid-Open Patent Publication No. 2003-69152 discloses an example of a multi-beam LD device. The Japanese Laid-Open Patent Publication No. 2003-69152 discloses a configuration including four active regions aligned at pitches of at least 16 μm that are at a predetermined height on a sapphire substrate and are parallel to the sapphire substrate.
The line pitch in a sub-scanning direction on the photoreceptor needs to be matched with an integral multiple of the resolution in the sub-scanning direction. When the above multiple-emission LD is used, the line pitch in the sub-scanning direction on the photoreceptor is determined uniquely by the disposition pitch of the emission points of the LD and the magnification in the sub-scanning direction from the LD to the photoreceptor.
However, the magnification in the sub-scanning direction from the LD to the photoreceptor is determined depending on an optical system that the optical scanning unit has and the disposition pitch of the emission points of the LD is determined by the semiconductor process thereof. That is, when the line pitch in the sub-scanning direction on the photoreceptor is tried to be matched with an integral multiple of the resolution in the sub-scanning direction as above, a fully satisfactory configuration can not always be obtained due to restrictions concerning specifications and configurations of devices and members used in the optical system or restrictions concerning specifications, etc., of an available LD.
For the demand to match the line pitch in the sub-scanning direction on the photoreceptor with an integral multiple of the resolution in the sub-scanning direction, a scheme is known according to which a multi-emission LD is rotated against the main scanning plane (that is, the main optical axis plain) and the direction of the disposition of emitting points of the LD is set to be slanted against the main scanning plane and, thereby, the line pitch in the sub-scanning direction on the photoreceptor is adjusted to be a desired value.
For example, when image forming is executed with 600 dpi that is an ordinary pixel density (resolution), the scanning line interval (the line pitch in the sub-scanning direction) D is 42.3 μm. In this case, when, for example, the LD device described in the above Japanese Laid-Open Patent Publication No. 2003-69152 is used, the emitting point interval is at least 16 μm. Therefore, to match the scanning line interval using this LD, it is necessary to set the sub-scanning line magnification of the scanning optical system to an integral multiple of 42.3/16 and, therefore, a problem has arisen that the above is a significant restriction on the optical design.
In this case, the LD device is rotated by an angle θ against the main optical axis plane (the main scanning plane) of the LD device and, thereby, an apparent emitting point pitch in the sub-scanning direction is equalized with the above necessary emitting point interval d.
An LD applied to a scanning optical system of an optical scanning unit can be generally an edge emitting LD in the viewpoint of the cost and the output power. The emission profile of an edge emitting LD is elliptic due to its structure. FIG. 10 diagrammatically depicts the emission profile of a laser beam 122 emitted from an edge emitting laser diode (LD) 101 having one emitting portion 121. The edge emitting LD 101 emits the laser beam 122 having the emission profile that is laterally elongated against the emitting portion 121 that is longitudinally elongated due to astigmatism thereof. An example of the divergence angle of the laser from the LD 101 is 30° in the long axis direction and 11° in the short axis direction of the elliptic profile.
The above phenomenon similarly occurs to the above multi-emitting point LD and profiles of the plurality of emitted light beams are respectively elliptic profiles. FIG. 11 diagrammatically depicts emission profiles of the laser beams 122 emitted from the edge emitting laser diode (LD) 101 having two emitting portions 121.
When the above multi-emission LD 101 is used and the LD 101 is rotated against the optical axis thereof to match the line pitch on the photoreceptor to a desired value, the emission profiles thereof also are rotated simultaneously as shown in FIG. 11.
In a laser scanning optical system utilizing a single-emitting point LD having one emitting portion, usually, a laser beam emitted by the LD is converted by a collimator lens into substantially parallel light and, thereafter, the beam is shaped by an aperture such that the shape of the beam is elongated in the main scanning direction.
The aperture is provided to determine the beam diameter in the main scanning direction on a photoreceptor by defining the width of the beam in the main scanning direction. In an aperture portion, when the width in the main scanning direction of the light beam is reduced, the beam diameter in the main scanning direction on the photoreceptor is enlarged.
At this time, to reduce eclipse by the aperture, the LD is disposed such that the long axis of the elliptic profile of the LD is substantially in the main scanning direction. FIG. 12 diagrammatically depicts the relation between the aperture and the emission profile in this case. In FIG. 12, “103” denotes the aperture and “122” denotes the laser beam having the elliptic emission profile.
For the multi-emitting point LD, as above, the rotation direction of the LD is determined to match the line pitch in the sub-scanning direction on the photoreceptor to a desired value.
When the LD is rotated, the long axis direction of the aperture and the long axis direction of the emission profile do not coincide with each other. In this case, the beam width may be smaller than the aperture width depending on the focal length f of the collimator lens.
FIG. 13 diagrammatically depicts the relation between the one laser beam 122 and the aperture 103.
In the above case, by increasing the focal length f of the collimator lens, the beam width in the main scanning direction at the position of the aperture can be increased to a length larger than the aperture width in the longitudinal direction. However, in this case, the beam width in the sub-scanning direction is increased together with that in the main scanning direction and, therefore, the eclipse by the aperture is also increased and the beam power passing through the aperture is decreased. That is, the utilization efficiency of the emitted laser beam is degraded and the transmission efficiency of the emitted energy is degraded.
Thereby, an LD with large power is necessary to secure the applied power on the photoreceptor and many problems have arisen such as increase of power consumption, occurrence of adverse effects due to heat dissipation, and delay of switching time specific to a high output power LD.
Because the power of the beam exiting from the aperture varies according to the rotation angle of the LD, a problem has arisen that the power is varied with the rotation of the LD when, for example, the rotation angle of the LD is corrected to correct the magnification that is varied by temperature variation.